The objective of this project is to evaluate the capabilities of various tissue lipases and esterases as early mediators in the interactions of animals with xenobiotics containing ester linkages. These enzymes are also used as models in studies of protein-ligand interactions. Recent studies have led to partial characterization of the enzyme in human plasma responsible for the hydrolysis of phthalate diesters (from blood bag plastics) to the more soluble monoesters. This esterase has been found to be distinguishable from both triglyceride lipase and lipoprotein lipase.